Search for a Solution of the Pioneer Anomaly

In 1972 and 1973 the Pioneer 10 and 11 missions were launched. They were the first to explore the outer solar system and achieved stunning breakthroughs in deep-space exploration. But beginning in about 1980 an unmodelled force of ∼8×10−8 cm s−2, directed approximately towards the Sun, appeared in the tracking signal. It later was unambiguously verified as being in the data and not an artefact. The cause remains unknown (although radiant heat remains a likely origin). With time more and more effort has gone into understanding this anomaly (and also possibly related effects). We review the situation and describe ongoing programmes to resolve the issue.